Inverter system

ABSTRACT

A unit power cell of an inverter system, according to one embodiment of the present invention, comprises: a first leg including first and fourth switching elements, which are connected in series to each other, second and third switching elements, which are connected in series with each other between a connection point of the first and second switching elements and a smoothing unit, and first, second, third and fourth diodes, which are inversely and respectively connected in parallel with the first, second, third and fourth switching elements; and a second leg connected in parallel with the first leg and including fifth and sixth switching elements, which are connected in series to each other, and fifth and sixth diodes, which are inversely and respectively connected in parallel with the fifth and sixth switching elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage of International ApplicationNo. PCT/KR2018/008109, filed on Jul. 18, 2018, which claims the benefitof earlier filing date and right of priority to Korean Application No.10-2017-0123403, filed on Sep. 25, 2017, and Korean Application No.10-2017-0123402, filed on Sep. 25, 2017 the contents of which are allhereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present disclosure relates to an inverter system, and moreparticularly, to an inverter system including an inverter having newtopology.

BACKGROUND OF THE INVENTION

High voltage inverter systems use input power sources with aroot-mean-square (RMS) line voltage of 600 V or more and are generallyused to operate a large-capacity motor with a capacity of hundreds of kWto tens of MW. High voltage inverter systems are generally used infields such as fans, pumps, compressors, retractors, hoists, andconveyors.

The inverter systems include a form of series-type multi-level inverter(cascade multi-level inverter) that generates three levels or more ofoutput voltage. Magnitude and a number of output voltage levels of theinverter system are determined based on a number of unit power cellsincluding multi-level inverter, and each of unit power cells uses anisolated input voltage.

In the inverter system, unit power cells of a plurality of unit powercells are connected electrically in series to form each of phases and amulti-phase output voltage of the inverter is determined based on a sumof output voltages of the unit power cells included in phases. In thiscase, the inverters included in each unit power cell may have varioustopologies.

FIG. 1 shows a configuration of a unit power cell including an inverterhaving a topology in related art.

Referring to FIG. 1, a unit power cell including an inverter havingtopology in related art includes a rectifier 102, a smoother 104, and aninverter 106 that synthesizes an output voltage.

The rectifier 102 receives two three-phase voltages output from an inputpower source. The rectifier 102 includes a plurality of diodes and avoltage magnitude of the rectified direct current (DC)-link isdetermined based on a difference between input power of the rectifier102 and output power of the unit power cell.

The output of the rectifier 102 is transferred to the smoother 104including two DC-link capacitors C1, C2 connected to each otherelectrically in series. The DC-link capacitors C1 and C2 function tosolve instantaneous power imbalance at an input/output terminal. In FIG.1, the capacitors C1 and C2 represent the same voltage of E.

The inverter 106 synthesizes the output voltage based on the DC voltageprovided through the rectifier 102 and the DC-link capacitors C1 and C2.As shown in FIG. 1, the inverter 106 is configured based on a T-typetopology in related art and includes a plurality of switching elementsS1 to S8 and a plurality of diodes D1 to D12.

The switching elements S1 to S8 included in the inverter 106 arerespectively connected to the corresponding diodes D1 to D8 electricallyin inverse-parallel. In the present disclosure, the ‘inverse parallel’between the switching element and the diode refers that a direction ofcurrent flowing through the diode and a direction of current flowingthrough the switching element when the switching element is turned onare opposite to each other.

The switching elements S1 and S5 and the switching elements S3 and S7 ofthe inverter 106 in related art shown in FIG. 1 are turned on and off ina complementary manner and the switching elements S2 and S6 and theswitching element S4 and S8 are turned on and turned off in acomplementary manner.

For example, in the case where the voltages of the DC-link capacitors C1and C2 are each E, when the switching element S1 and the switchingelement S2 are turned on, the switching element S3 and the switchingelement S4 are turned off, and at this time, an output pole voltage (Vu)becomes E.

In addition, when the switching element S1 and the switching element S3are turned on, the switching element S2 and the switching element S4 areturned off, and in this case, the output pole voltage becomes 0.Similarly, when the switching element S1 and the switching element S2are turned off, the switching element S3 and the switching element S4are turned on, and in this case, the output pole voltage becomes −E.

Similarly, three levels of pole voltages Vv are output based on thecomplementary turn-on and turn-off operation of the switching elementsS5 to S8. Based on a combination of the two output pole voltages outputas described above, the unit power cell in FIG. 1 may represent fivevoltage levels of 2E, E, 0, −E, and −2E.

However, the inverter having the topology in related art as shown inFIG. 1 includes too many switching elements and diodes. As describedabove, when each of unit power cells includes many elements, apossibility of failure of each of elements is increased as the number ofused elements is increased. This increase in a possibility of failureresults in degraded reliability of the high voltage inverter systemincluding the inverter as shown in FIG. 1

In particular, as the number of switching elements is increased, anamount of heat generated by repeating the switching operation(turn-on/turn-off) of the switching elements is increased. The increasein the amount of heat generation causes increase in the possibility offailure of the unit power cell and the inverter system.

In addition, when the inverter including excessive elements as shown inFIG. 1 is used, there is a problem that the magnitude and the volume ofthe high-voltage inverter system are increased.

BRIEF SUMMARY OF THE INVENTION

The present disclosure provides an inverter and an inverter system towhich new topology is applied, which may reduce a possibility of failurethereof by reducing a number of internal elements compared to aninverter having topology in related art.

The present disclosure also provides an inverter system having a reducedsize and volume compared to an inverter system in the related art byreducing the number of internal elements compared to the inverter havingthe topology in related art.

The objects of the present disclosure are not limited to theabove-mentioned objects, and the other objects and the advantages of thepresent disclosure which are not mentioned can be understood by thefollowing description, and more clearly understood by the embodiments ofthe present disclosure. It will be also readily seen that the objectsand the advantages of the present disclosure may be realized by featuresdescribed in the patent claims and a combination thereof.

According to an embodiment of the present disclosure, an inverter systemincludes a phase shift transformer configured to convert and output aphase and magnitude of a voltage input from a power supply and aplurality of unit power cells configured to output a phase voltage basedon voltage output from the phase shift transformer, and the unit powercell includes a first leg and a second leg. The first leg includes afirst switching element and a fourth switching element connected to eachother electrically in series, a second switching element and a thirdswitching element connected to each other electrically in series betweena connection point between the first switching element and the fourthswitching element and a smoother, and a first diode, a second diode, athird diode, and a fourth diode respectively connected to the firstswitching element, the second switching element, the third switchingelement, and the fourth switching element electrically ininverse-parallel. The second leg includes a fifth switching element anda sixth switching element connected to each other electrically in seriesand a fifth diode and a sixth diode respectively connected to the fifthswitching element and the sixth switching element electrically ininverse-parallel and is connected to the first leg electrically inparallel.

In addition, according to another embodiment of the present disclosure,the inverter system includes a phase shift transformer configured toconvert and output the phase and the magnitude of the voltage input fromthe power supply and a plurality of unit power cells configured tooutput a phase voltage based on the voltage output from the phase shifttransformer and the unit power cell includes a first leg and a secondleg. The first leg includes a first switching element, a secondswitching element, a third switching element, and a fourth switchingelement connected to one another electrically in series, a first diode,a second diode, a third diode, and a fourth diode respectively connectedto the first switching element, the second switching element, the thirdswitching element, and the fourth switching element electrically ininverse-parallel, and a seventh diode and an eighth diode connected toeach other electrically in series between a connection point between thefirst switching element and the second switching element and aconnection point between the third switching element and the fourthswitching element and the second leg includes a fifth switching elementand a sixth switching element connected to each other electrically inseries and a fifth diode and a sixth diode respectively connected to thefifth switching element and the sixth switching element electrically ininverse-parallel and is connected to the first leg electrically inparallel.

According to the present disclosure, inverters and inverter systems towhich new topology is applied have an advantage in that a possibility offailure is reduced due to reduction in a number of internal elementscompared to an inverter having topology in related art.

In addition, according to the present disclosure, the inverter systemhas an advantage in that a size and a volume is reduced compared to theinverter system in related art by reducing the number of internalelements compared to the inverter having the topology in related art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a configuration of a unit power cell including an inverterhaving topology in related art.

FIG. 2 shows a configuration of an inverter system according to anembodiment of the present disclosure.

FIG. 3 is a circuit diagram showing a unit power cell included in aninverter system according to an embodiment of the present disclosure.

FIG. 4 shows waveforms of output pole voltages determined based on turnon/turn off states of switching elements of an inverter of the unitpower cell shown in FIG. 3.

FIGS. 5 to 7 show current flow determined based on turn-on and turn-offstates of switching elements of the inverter of the unit power cellshown in FIG. 3.

FIG. 8 shows a current flow determined when a unit power cell outputs apole voltage according to another embodiment of the present disclosure.

FIG. 9 is a circuit diagram showing a unit power cell included in aninverter system according to another embodiment of the presentdisclosure.

FIGS. 10 to 13 show current flow determined based on turn-on andturn-off states of switching elements of the inverter of the unit powercell shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

The above objects, features, and advantages will be described in detailwith reference to the accompanying drawings, whereby those skilled inthe art to which the present disclosure pertains may easily implementthe technical idea of the present disclosure. In describing the presentdisclosure, when it is determined that the detailed description of theknown technology related to the present disclosure may unnecessarilyobscure the gist of the present disclosure, the detailed descriptionwill be omitted. Hereinafter, exemplary embodiments of the presentdisclosure will be described in detail with reference to theaccompanying drawings. In the drawings, the same reference numerals areused to indicate the same or similar components.

FIG. 2 shows a configuration of an inverter system according to anembodiment of the present disclosure.

According to an embodiment of the present disclosure, as shown in FIG.2, an inverter system 204 converts power input from a power supply 202and provides the power to a three-phase motor 210. For example, thepower supply 202 may supply an inverter system 204 with three-phasepower having a root-mean-square (RMS) voltage of 600 V or more. Inaddition, the three-phase motor 210 may be an induction motor or asynchronous motor as examples of a load connected to the inverter system204. According to embodiments, the load other than the three-phase motor210 may be connected to the inverter system 204.

Referring back to FIG. 2, the inverter system 204 includes a phase shifttransformer 206 and a plurality of unit power cells 20 a 1, 20 a 2, 20 b1, 20 b 2, 20 c 1, and 20 c 2.

The phase shift transformer 206 may convert the phase and magnitude ofthe voltage input from the power supply 202 and provide the voltage tothe plurality of unit power cells 20 a 1, 20 a 2, 20 b 1, 20 b 2, 20 c1, and 20 c 2. Total harmonic distortion (THD) of the input current maybe improved through the phase shift.

The unit power cells 20 a 1, 20 a 2, 20 b 1, 20 b 2, 20 c 1, and 20 c 2receive the output voltage output from the phase shift transformer 206and output a phase voltage suitable for a load, for example, athree-phase motor 210.

In FIG. 2, the unit power cells 20 a 1, 20 a 2, 20 b 1, 20 b 2, 20 c 1,and 20 c 2 output three-phase voltages for the three-phase motor 210.That is, two unit power cells 20 a 1 and 20 a 2 connected to each otherelectrically in series output a-phase voltage, and two unit power cells20 b 1 and 20 b 2 connected to each other electrically in series outputb-phase voltage, and two unit power cells 20 c 1 and 20 c 2 connected toeach other electrically in series output c-phase voltage. FIG. 2 showsan example of two unit power cells being electrically connected to eachother for each phase, but the number of unit power cells connected toeach other for each phase may vary depending on the output voltage ofthe inverter system 204.

The phase voltages output by the unit power cells 20 a 1, 20 a 2, 20 b1, 20 b 2, 20 c 1, 20 c 2 of the inverter system 204 shown in FIG. 2have the same magnitude and the phases are different from one another by120 degrees. In addition, the number of unit power cells of the invertersystem 204 may be reduced and the THD of the output voltage and avoltage change rate (dv/dt) may be improved through various switchingmethods.

Configurations and operation of a unit power cell including an inverterhaving new topology according to the present disclosure are describedbelow in detail with reference to FIGS. 3 to 7.

FIG. 3 is a circuit diagram showing a unit power cell of an invertersystem according to an embodiment of the present disclosure.

According to an embodiment of the present disclosure, referring to FIG.3, a unit power cell of an inverter system includes a rectifier 302, asmoother 304, and an inverter 306 that synthesizes an output voltage.

The rectifier 302 receives two three-phase voltages output from an inputpower source. The rectifier 302 includes a plurality of diodes andmagnitude of the rectified DC-link voltage is determined based on adifference between input power of the rectifier 302 and output power ofthe unit power cell.

The output of the rectifier 302 is transmitted to the smoother 304including two DC-link capacitors C1 and C2 connected to each otherelectrically in series. The DC-link capacitors C1 and C2 function tosolve instantaneous power imbalance at the input/output terminal.

In the following embodiments, it is assumed that the magnitude of thevoltage represented by each of the capacitors C1 and C2 is E. Forreference, the magnitude of the voltage represented by each of thecapacitors C1 and C2 may vary according to the embodiments.

The inverter 306 synthesizes the output voltage based on the DC voltageprovided through the rectifier 302 and the DC-link capacitors C1 and C2.As shown in FIG. 3, the inverter 306 includes a first leg 308 and asecond leg 310 connected to each other electrically in parallel.

The first leg 308 may include a first switching element S1 and a fourthswitching element S4 connected to each other electrically in series, anda second switching element S2 and a third switching element S2 connectedto each other electrically in series between a connection point N2between the first switching element S1 and the fourth switching elementS4 and a connection point N1 of the rectifier 304. Further, as shown inFIG. 3, the first leg 308 includes the first diode D1, the second diodeD2, the third diode D3, and the fourth diode D4 respectively connectedto the first switching element S1, the second switching element S2, thethird switching element S3, and the fourth switching element S4electrically in inverse-parallel.

The first diode D1 and the second diode D2 included in the first leg 308are electrically connected to each other in a same direction. Inaddition, the third diode D3 and the fourth diode D4 are electricallyconnected to each other in a same direction.

Referring back to FIG. 3, the second leg 310 includes a fifth switchingelement S5 and a sixth switching element S6 connected to each otherelectrically in series, and a fifth diode D5 and a sixth diode D6respectively connected to the fifth switching element S5 and the sixthswitching element S6 electrically in inverse-parallel. The fifth diodeD5 and the sixth diode D6 included in the second leg 310 areelectrically connected to each other in the same direction.

The inverter 306 having the above configuration may output pole voltagehaving four levels, for example, a first voltage level, a second voltagelevel, a third voltage level, and a fourth voltage through the switchingoperation of the switching elements S1 to S6 described below.

The inverter 106 in related art shown in FIG. 1 includes eight switchingelements and twelve diodes, whereas the inverter 306 of the unit powercell of the present disclosure shown in FIG. 3 includes six switchingelements and sixth diodes. As described above, the unit power cellaccording to the present disclosure has less number of switchingelements than that of the unit power cell in related art to relativelyreduce the possibility of failure and reduce the size and the volume ofthe unit power cell through the arrangement of the switching elementscompared to the unit power cell in related art.

FIG. 4 shows waveforms of output pole voltages determined based on turnon/turn off states of switching elements of the inverter of the unitpower cell shown in FIG. 3.

In FIG. 4, V_(g1) to V_(g6) refer to gate signals applied to gateterminals of the switching elements S1 to S6, respectively. That is,when gate signals V_(g1) to V_(g6) are displayed in black shades, thecorresponding switching elements S1 to S6 are turned on. Otherwise, theswitching elements S1 to S6 are turned off.

In addition, +E, 0, −E displayed at the top of FIG. 4 indicate themagnitudes of phase voltages.

According to the present disclosure, as shown in FIG. 4, each of phases(U, V) of a unit power cell may output three levels (+E, 0, −E) of phasevoltages based on a turn on/off state through switching operation ofeach of switching elements. The unit power cell may represent the polevoltages V_(UV) having four levels (+2E, +E, −E, and −2E) based on thecombination of the U-phase voltage V_(UN1) and V-phase voltage V_(VN1).

Output of the phase voltages V_(UN1) and V_(VN1) determined through theswitching operation of each of switching elements and the pole voltageV_(UV) of the unit power cell determined based on a combination of thephase voltages V_(UN1) and V_(VN1) is described below in detail withreference to FIGS. 4 and 5 to 8.

FIGS. 5 to 8 respectively show a current flow determined based on aturn-on and turn-off states of switching elements of the inverter of theunit power cell shown in FIG. 3.

First, FIG. 5 shows a current flow 502 determined when a unit power celloutputs a pole voltage having a first voltage level, that is, +2E.

Referring to FIGS. 4 and 5, when the first switching element S1 and thesecond switching element S2 included in the inverter 306 are turned on,the U-phase voltage V_(UN1) represents +E. In addition, when the sixthswitching element S6 is turned on, the V-phase voltage V_(VN1)represents −E. Accordingly, the pole voltage V_(UV) of the unit powercell, which corresponds to a difference (V_(UN1)−V_(VN1)) between theU-phase voltage V_(UN1) and the V-phase voltage V_(VN1), satisfiesequation of +E−(−E)=+2E.

As a result, when the first switching element S1, the second switchingelement S2, and the sixth switching element S6 included in the inverter306 are turned on and the third switching element S3, the fourthswitching element S4, and the fifth switching element S5 are turned off,the pole voltage V_(UV) of the unit power cell is represented by thefirst voltage level, that is, +2E. In this case, as shown in FIG. 5, thecurrent flows through the DC-link capacitors C1 and C2, the firstswitching element S1, and the sixth switching element S6 (see thecurrent flow 502).

FIG. 6 shows a current flow 602 determined when a unit power celloutputs a pole voltage having a second voltage level, that is, +E.

Referring to FIGS. 4 and 6, when a second switching element S2 and athird switching element S3 included in an inverter 306 are turned on, aU-phase voltage V_(UN1) represents zero. In addition, based on a six andthe switching element S6 is turned on, a V-phase voltage V_(VN1)represents −E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding to the difference (V_(UN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of0−(−E)=+E.

As a result, when the second switching element S2, the third switchingelement S3, and the sixth switching element S6 included in the inverter306 are turned on and the first switching element S1, the fourthswitching element S4, and the fifth switching element S5 are turned off,the pole voltage V_(UV) of the unit power cell is represented by asecond voltage level, that is, +E. In this case, as shown in FIG. 6, thecurrent flows through the DC-link capacitor C2, the third switchingelement S3, the second diode D2, and the sixth switching element S6 (seethe current flow 602).

Next, FIG. 7 shows a current flow 702 determined when a unit power celloutputs a pole voltage having a third voltage level, that is, −E.

Referring to FIGS. 4 and 7, when a second switching element S2 and thethird switching element S3 included in an inverter 306 are turned on, aU-phase voltage V_(UN1) represents 0. In addition, when a fifthswitching element S5 is turned on, a V-phase voltage V_(VN1) represents+E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding to a difference (V_(UN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of0−(+E)=−E.

As a result, when the second switching element S2, the third switchingelement S3, and the fifth switching element S5 included in the inverter306 are turned on and the first switching element S1, the fourthswitching element S4, and the sixth switching element S6 are turned off,the pole voltage V_(UV) of the unit power cell is represented by a thirdvoltage level, that is, −E. In this case, as shown in FIG. 7, thecurrent flows through the DC-link capacitor C1, the third switchingelement S3, the second diode D2, and the fifth switching element S5 (seethe current flow 702).

Next, FIG. 8 shows a current flow 802 determined when a unit power celloutputs a pole voltage having a fourth voltage level, that is, −2E.

Referring to FIGS. 4 and 8, when a third switching element S3 and afourth switching element S4 included in an inverter 306 are turned on, aU-phase voltage V_(UN1) represents −E. In addition, when a fifthswitching element S5 is turned on, a V-phase voltage V_(VN1) represents+E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding a difference (V_(UN1)−V_(VN1)) between the U-phase voltageV_(UN1) and the V-phase voltage V_(VN1) satisfies equation of−E−(+E)=−2E.

As a result, when the third switching element S3, the fourth switchingelement S4, and the fifth switching element S5 included in the inverter306 are turned on and a first switching element S1, a second switchingelement S2, and a sixth switching element S6 are turned off, the polevoltage V_(UV) of the unit power cell is represented by a fourth voltagelevel, that is, −2E. In this case, as shown in FIG. 8, the current flowsthrough DC-link capacitors C1 and C2, the fifth switching element S5,and the fourth switching element S4 (see the current flow 802).

FIG. 9 is a circuit diagram showing a unit power cell included ininverter system according to another embodiment of the presentdisclosure.

According to another embodiment of the present disclosure, referring toFIG. 9, the unit power cell included in the inverter system includes arectifier 902, a smoother 904, and an inverter 906 that synthesizes anoutput voltage.

The rectifier 902 receives two three-phase voltages output from an inputpower source. The rectifier 902 includes a plurality of diodes andmagnitude of the rectified DC-link voltage is determined based on adifference between input power of the rectifier 902 and output power ofthe unit power cell.

The output of the rectifier 902 is transmitted to the smoother 904including two DC-link capacitors C1 and C2 connected to each otherelectrically in series. The DC-link capacitors C1 and C2 function tosolve instantaneous power imbalance at the input/output terminal.

In the following embodiments, it is assumed that the magnitude of thevoltage represented by each of the capacitors C1 and C2 is E. Forreference, the magnitude of the voltage represented by each of thecapacitors C1 and C2 may vary according to the embodiment.

The inverter 906 synthesizes the output voltage based on the DC voltageprovided through the rectifier 902 and the DC-link capacitors C1 and C2.As shown in FIG. 9, the inverter 906 includes a first leg 908 and asecond leg 910 connected to each other electrically in parallel.

The first leg 908 includes a first switching element S1, a secondswitching element S2, a third switching element S3, and a fourthswitching element S4 connected to one another electrically in series. Inaddition, as shown in FIG. 9, the first leg 908 includes the first diodeD1, the second diode D2, the third diode D3, and the fourth diode D4respectively connected to the first switching element S1, the secondswitching element S2, the third switching element S3, and the fourthswitching element S4 electrically in inverse-parallel.

In addition, the first leg 908 includes a seventh diode D7 and an eighthdiode D8 connected to each other electrically in series between aconnection point N1 between the first switching element S1 and thesecond switching element S2 and a connection point N2 between the thirdswitching element S3 and the fourth switching element S4. A connectionpoint N4 between the seventh diode D7 and the eighth diode D8 iselectrically connected to the connection point N3 between the DC-linkcapacitors C1 and C2.

The first diode D1, the second diode D2, the third diode D3, and thefourth diode D4 included in the first leg 908 are electrically connectedto one another in the same direction. In addition, the seventh diode D7and the eighth diode D8 included in the first leg 908 are electricallyconnected to each other in the same direction.

Referring back to FIG. 9, the second leg 910 includes a fifth switchingelement S5 and a sixth switching element S6 connected to each otherelectrically in series and a fifth diode D5 and a sixth diode D6respectively connected to the fifth switching element S5 and the sixthswitching element S6 electrically in inverse-parallel. The fifth diodeD5 and the sixth diode D6 included in the second leg 910 areelectrically connected to each other in the same direction.

The inverter 906 having the above configuration may output the polevoltage having four levels, for example, a first voltage level, a secondvoltage level, a third voltage level, and a fourth voltage through theswitching operation of the switching elements S1 to S6 described below.

The inverter 106 in related art shown in FIG. 1 includes eight switchingelements and twelve diodes, whereas the inverter 906 of the unit powercell of the present disclosure shown in FIG. 9 includes six switchingelements and eight diodes. As described above, the unit power cellaccording to the present disclosure has less number of switchingelements than that of the unit power cell in the related art to therebyrelatively reduce the failure possibility and reduce the size and thevolume of the unit power cell due to the arrangement of the switchingelements compared to the unit power cell in related art. Accordingly,the possibility of failure, the size, and the volume of the invertersystem 204 including the unit power cell in FIG. 9 are reduced comparedto the inverter system in related art.

Output of the phase voltages V_(UN1) and V_(VN1) determined based onswitching operation of the switching elements and pole voltage V_(UV) ofthe unit power cell determined based on a combination of phase voltagesV_(UN1) and V_(VN1) are described below in detail with reference toFIGS. 4 and 10 to 13.

FIGS. 10 to 13 show current flow determined based on turn-on andturn-off states of switching elements of the inverter of the unit powercell shown in FIG. 9.

First, FIG. 10 shows a current flow 502 determined when a unit powercell outputs a pole voltage having a first voltage level, that is, +2E.

Referring to FIGS. 4 and 10, based on a first switching element S1 and asecond switching element S2 included in an inverter 906 being turned on,a U-phase voltage V_(UN1) represents +E. In addition, when a sixthswitching element S6 is turned on, a V-phase voltage V_(VN1) represents−E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding to a difference (V_(UN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of+E−(−E)=+2E.

As a result, when the first switching element S1, the second switchingelement S2, and the sixth switching element S6 included in the inverter906 are turned on and a third switching element S3, a fourth switchingelement S4, and a fifth switching element S5 are turned off, the polevoltage V_(UV) of the unit power cell is represented by a first voltagelevel, that is, +2E. In this case, as shown in FIG. 10, the currentflows through DC-link capacitors C1 and C2, the first switching elementS1, the second switching element S2, and the sixth switching element S6(see the current flow 502).

Next, FIG. 11 shows a current flow 602 determined when a unit power celloutputs a pole voltage having a second voltage level, that is, +E.

Referring to FIGS. 4 and 11, when a second switching element S2 and athird switching element S3 included in an inverter 906 are turned on, aU-phase voltage V_(UN1) represents 0. In addition, when a sixthswitching element S6 is turned on, a V-phase voltage V_(VN1) represents−E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding to a difference (V_(UN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of0−(−E)=+E.

As a result, when the second switching element S2, the third switchingelement S3, and the sixth switching element S6 included in the inverter906 are turned on and the first switching element S1 and the fourthswitching element S4, and the fifth switching element S5 are turned on,the pole voltage V_(UV) of the unit power cell is represented by asecond voltage level, that is, +E. In this case, as shown in FIG. 11,the current flows through DC-link capacitor C2, a seventh diode D7, thesecond switching element S2, and the sixth switching element S6 (see thecurrent flow 602).

Next, FIG. 12 shows a current flow 702 determined when a unit power celloutputs a pole voltage having a third voltage level, that is, −E.

Referring to FIGS. 4 and 12, when a second switching element S2 and athird switching element S3 included in an inverter 906 are turned on, aU-phase voltage V_(UN1) represents 0. In addition, when the fifthswitching element S5 is turned on, a V-phase voltage V_(VN1) represents+E. Accordingly, the pole voltage V_(UV) of the unit power cellcorresponding to a difference (V_(UN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of0−(+E)=−E.

As a result, when the second switching element S2, the third switchingelement S3, and the fifth switching element S5 included in the inverter906 are turned on and the first switching element S1 and the fourthswitching element S4, and the sixth switching element S6 are turned off,the pole voltage V_(UV) of the unit power cell is represented by a thirdvoltage level, that is, −E. In this case, as shown in FIG. 12, thecurrent flows through the DC-link capacitor C1, the seventh diode D7,the second switching element S2, and the fifth switching element S5 (seethe current flow 702).

Next, FIG. 13 shows a current flow 802 determined when a unit power celloutputs a pole voltage having a fourth voltage level, that is, −2E.

Referring to FIGS. 4 and 13, when a third switching element S3 and afourth switching element S4 included in an inverter 906 are turned on, aU-phase voltage V_(UN1) represents −E. In addition, when a fifthswitching element S5 is turned on, a V-phase voltage V_(VN1) represents+E. Accordingly, a pole voltage V_(UV) of the unit power cellcorresponding to a difference (V_(VN1)−V_(VN1)) between the U-phasevoltage V_(UN1) and the V-phase voltage V_(VN1) satisfies equation of−E−(+E)=−2E.

As a result, when the third switching element S3, the fourth switchingelement S4, and the fifth switching element S5 included in the inverter906 are turned on and a first switching element S1, a second switchingelement S2, and a sixth switching element S6 are turned off, the polevoltage V_(UV) of the unit power cell is represented by a fourth voltagelevel, that is, −2E. In this case, as shown in FIG. 13, the currentflows through DC-link capacitors C1 and C2, the third switching elementS3, the fourth switching element S4, and the fifth switching element S5(see the current flow 802).

As described above, the unit power cell including the inverter havingthe new topology of the present disclosure may include less number ofelements than that of the power unit cell in related art to output thepole voltages having four levels. As described above, the number ofelements may be reduced to reduce the failure possibility of the unitpower cell and the inverter system to thereby improve reliability andreduce the size, the volume, and production costs of the unit power celland the inverter system.

In particular, as the number of switching elements used for the inverteris reduced, the amount of heat generated by the switching elements isalso reduced compared inverter systems in related art. The possibilityof failure of the entire inverter system is reduced due to the reductionin the amount of generated heat. In addition, the size of additionalcomponents, for example, heat sinks, to solve heat generation of theinverter system may be reduced, which helps to reduce the size andvolume of the inverter system.

Various substitutions, modifications, and changes can be made within arange that does not deviate from the technical idea of the presentdisclosure for a person having ordinary skill in the art to which thepresent disclosure pertains, and thus, the above-mentioned presentdisclosure is not limited to the above-mentioned embodiments andaccompanying drawings.

The invention claimed is:
 1. An inverter system, comprising: a phaseshift transformer configured to convert and output a phase and magnitudeof a voltage input from a power supply; a plurality of unit power cellsconfigured to output a phase voltage based on a voltage output from thephase shift transformer, wherein each unit power cell comprises: a firstleg comprising a first switching element and a fourth switching elementconnected to each other electrically in series, a second switchingelement and a third switching element connected to each otherelectrically in series between a connection point between the firstswitching element and the fourth switching element and a smoother, and afirst diode, a second diode, a third diode, and a fourth dioderespectively connected to the first switching element, the secondswitching element, the third switching element, and the fourth switchingelement electrically in inverse-parallel; and a second leg comprising afifth switching element and a sixth switching element connected to eachother electrically in series and a fifth diode and a sixth dioderespectively connected to the fifth switching element and the sixthswitching element electrically in inverse-parallel and is connected tothe first leg electrically in parallel, wherein a pole voltage of theunit power cell represents a first voltage level when the firstswitching element, the second switching element, and the sixth switchingelement are turned on and the third switching element, the fourthswitching element, and the fifth switching element are turned off. 2.The inverter system of claim 1, wherein a pole voltage of the unit powercell represents a second voltage level when the second switchingelement, the third switching element, and the sixth switching elementare turned on and the first switching element, the fourth switchingelement, and the fifth switching element are turned off.
 3. The invertersystem of claim 1, wherein a pole voltage of the unit power cellrepresents a third voltage level when the second switching element, thethird switching element, and the fifth switching element are turned onand the first switching element, the fourth switching element, and thesixth switching element are turned off.
 4. The inverter system of claim1, wherein a pole voltage of the unit power cell represents a fourthvoltage level when the third switching element, the fourth switchingelement, and the fifth switching element are turned on and the firstswitching element, the second switching element, and the sixth switchingelement are turned off.
 5. The inverter system of claim 1, wherein thesecond diode and the third diode are electrically connected to eachother in different directions from each other.
 6. The inverter system ofclaim 1, wherein the first diode, the fourth diode, the fifth diode, andthe sixth diode are electrically connected to one another in a samedirection.
 7. An inverter system, comprising: a phase shift transformerconfigured to convert and output a phase and magnitude of a voltageinput from a power supply; and a plurality of unit power cellsconfigured to output a phase voltage based on a voltage output from thephase shift transformer; wherein each unit power cell comprises a firstleg and a second leg and the first leg comprises a first switchingelement, a second switching element, a third switching element, and afourth switching element, a first diode, a second diode, a third diode,and a fourth diode respectively connected to the first switchingelement, the second switching element, the third switching element, andthe fourth switching element electrically in inverse-parallel, a seventhdiode and an eighth diode connected to each other electrically in seriesbetween a connection point between the first switching element and thesecond switching element and a connection point between the thirdswitching element and the fourth switching element and the second legcomprises a fifth switching element and a sixth switching elementconnected to each other electrically in series and a fifth diode and asixth diode respectively connected to the fifth switching element andthe sixth switching element electrically in inverse-parallel and isconnected to the first leg electrically in parallel, and wherein a polevoltage of the unit power cell represents a first voltage level when thefirst switching element, the second switching element, and the sixthswitching element are turned on and the third switching element, thefourth switching element, and the fifth switching element are turnedoff.
 8. The inverter system of claim 7, wherein a pole voltage of theunit power cell represents a second voltage level when the secondswitching element, the third switching element, and the sixth switchingelement are turned on and the first switching element, the fourthswitching element, and the fifth switching element are turned off. 9.The inverter system of claim 7, wherein a pole voltage of the unit powercell represents a third voltage level when the second switching element,the third switching element, and the fifth switching element are turnedon and the first switching element, the fourth switching element, andthe sixth switching element are turned off.
 10. The inverter system ofclaim 7, wherein a pole voltage of the unit power cell represents afourth voltage level when the third switching element, the fourthswitching element, and the fifth switching element are turned on and thefirst switching element, the second switching element, and the sixthswitching element are turned off.
 11. The inverter system of claim 7,wherein the first diode, the second diode, the third diode, the fourthdiode, the fifth diode, and the sixth diode are connected to one anotherin a same direction.
 12. The inverter system of claim 7, wherein theseventh diode and the eighth diode are electrically connected to eachother in a same direction.